My Sohal is VERY attactive....probably the most active of all of my fish and I have a Regal and Naso tang (and assorted others). It appears that he doesn't "sleep" like the other fish as he swims constantly in and out of the rocks. Based on that....he would need to eat more than the other fish. Mine is HUGE and GORGEOUS. (my fave fish.....though he is a bit of an A-hole at times). He is the more aggressive fish, but doesn't bully, just flexes his muscles and will chase but not nip.
I never use the clips for Nori, it takes longer to get the clip situated than it does for them to rip the Nori to shreds.
Try crumbling up the Nori and spreading it in the water. Tangs need algae, its their main food source in the wild. They love it. Crumbling the food will make it impossible for your Purple to be a jerk and keep it all for himself. He will be too busy chasing the bits and eating to worry about what anyone else is doing.